Fluids (e.g., oil, water, gas) may exist in a variety of materials, including geological formations. These fluids are often recovered using a well, or a borehole cut into the formation. During exploration and recovery operations, it is therefore useful to determine the characteristics of the formation in which the fluids reside.
Gamma ray measurements can be used to determine certain formation characteristics, such as the formation density, neutron porosity, and photoelectric factor. Knowledge of these characteristics can help geologists decide what type of rock makes up the formation (e.g., limestone, sandstone). To make such determinations, a source of neutrons that propagate into and react with the formation to produce gamma rays can be used. While readily available, chemical (e.g., radioisotope) sources of neutrons present a potential safety hazard and have associated logistical complications.
Besides finding a source that is safe and effective, other difficulties exist when attempting to determine formation characteristics. For example, some methods depend on accurate knowledge of the neutron generator output and/or calibration to open-hole log data. Other methods that operate to remove sensitivity to absolute neutron output do not take into account both neutron and gamma transport effects when measuring gamma flux at the detector. Still other methods fail to make provision for many variables that affect measurements, including standoff and variations in mud properties.